1. Field of the Invention
This invention relates to a solenoid valve and, more specifically, to a solenoid valve for use in conjunction with an air gun which is utilized in seismic exploration.
2. Brief Description of the Prior Art
Air guns are well known in the field of seismic exploration, such air guns being used to provide a seismic wave which penetrates the earth's crust and is reflected from the various strata therein. These reflected waves are analyzed to provide information as to the content and location of the subterranean strata from which the reflection takes place.
In the operation of an air gun of the above noted type, as exemplified by the patent of Chelminski (4,240,518), the solenoid valve acts to trip a shuttle valve which, in turn, fires the gun. The rate of discharge of the high pressure solenoid valve air determines the speed of shuttle release. Therefore, it is extremely important that the solenoid valve discharge air flow rate be maximized. The importance of a fast discharge rate is evidenced from the graph of FIG. 1 which is a comparison of the pressure supplied by a valve which provides full area flow rapidly (full flow) as opposed to a prior art valve which gradually increases the flow rate until full flow is achieved. It can be seen that the time window (delta t1) during which the pressure is in the gun triggering pressure range is much shorter for a system wherein full flow is rapidly achieved as compared to the time window (delta t2) of the conventional valve wherein full flow is achieved slowly. This means that the more rapidly a valve can achieve full flow, the smaller will be the triggering pressure range window, thereby increasing the accuracy of determining the actual triggering time and improving the likelihood of repeatability. It is therefore apparent that the rise time of the solenoid valve pressure is very important in reducing the firing time variation (t) of the air gun. The slower the pressure rise time, the greater the air gun firing time variation.
A reason for the relatively slow pressure rise time in conventional air gun solenoid valves is that the plunger slides on a center stem and gradually uncovers small drilled port holes in the stem as it travels. Uncovering of these holes provides a flow path for the discharging air, thereby gradually increasing the air flow rate from valve inlet to valve outlet. Due to the location of the stem seal, the plunger must travel a short distance before the ports begin to open, thus delaying the firing of the air gun. As the ports are continually opened, the flow area increases until it equals the flow area of the central hole of the stem. At that time, the flow area becomes the constant flow area of the stem holes.
It is readily apparent from the above discussion that improved accuracy in timing of gun firing can be achieved by providing an air gun capable of rapidly achieving the maximum air flow rate capability of the solenoid valve.